As bmike's answer points out, aside from digging through the projects source to determine what executables they install, there's no good way to determine what commands come with a given formula before installing it.

After a formula is installed, running

brew unlink --dry-run formula | grep "$(brew --prefix)/bin"

is a workable option now that --dry-run is available for brew unlink.

Before that was added I wrote an external command called brew executables that still has some benefits over the above (mainly in formatting and handling some links a bit differently). I'll include a simplified (and probably non-working, due to missing some variable assignments) version of it here:

version_in_use=$(echo "$brew_info" | grep "$HOMEBREW_PREFIX.*\*$" | sed -e "s|.*$formula/\([^ ]*\).*|\1|i")

cd "$HOMEBREW_CELLAR/$formula/$version_in_use" 
for dir in `find . -type d -mindepth 1 -maxdepth 1 -name "bin"`; do
    for file in `find "$dir" -type f -o -type l -perm +111`
    do
            filename=$(basename $file)
            echo $filename
    done        
done

In short, it pulls the list of executables out of $(brew --prefix)/$formula/$version_in_use/bin. The version on my GitHub is a bit more fleshed out, including some added ability to identify/indicate when there are commands that link to each other in this bin directory.

Homebrew cleanup does not have a -k flag, nor is there any way to get Homebrew use the cleanup command to skip specific formulas.

There was a feature request in the Github Homebrew project, but it was rejected.

Create Your Own Tap

The advice then was to create your own tap. Since you are simply creating another version, use brew edit [formula] to modify the existing formula.

See for example the homebrew/php/php54 homebrew/php/php55 and homebrew/php/php56 formulas.

Create @ version formula

Homebrew itself supports multiple versions of some formulas, especially for widely used programming languages. For example, there are multiple @ versions of Ruby

$ brew search /^ruby*/
rubberband ✔        ruby-completion     ruby@1.9 ✔          ruby@2.2
ruby ✔              ruby-install        ruby@2.0            ruby@2.3
ruby-build ✔        ruby@1.8            ruby@2.1
homebrew/portable/portable-ruby          homebrew/portable/portable-ruby@2.2

Here is the ruby@1.9 formula

class RubyAT19 < Formula
  desc "Powerful, clean, object-oriented scripting language"
  homepage "https://www.ruby-lang.org/"
  url "https://cache.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p551.tar.bz2"
  sha256 "b0c5e37e3431d58613a160504b39542ec687d473de1d4da983dabcf3c5de771e"
  revision 4

  bottle do
    sha256 "3db7c4d297998e6f53b10e35626bdf94f8db0836778ce8ef943846e1d5b35fbc" => :sierra
    sha256 "9c5dd40c620a597405e3c2d458bc9772ad1c661acf501b65f9e9d8871c300f7f" => :el_capitan
    sha256 "9334e46da0b55e3ea20bba27c49352b8b9f95674a5dea1691b5fd04625f23eed" => :yosemite
  end

  keg_only :versioned_formula

  option "with-suffix", "Suffix commands with '193'"
  option "with-doc", "Install documentation"
  option "with-tcltk", "Install with Tcl/Tk support"

  depends_on "pkg-config" => :build
  depends_on "readline" => :recommended
  depends_on "gdbm" => :optional
  depends_on "libyaml"
  depends_on "openssl"
  depends_on :x11 if build.with? "tcltk"

  def install
    args = %W[
      --prefix=#{prefix}
      --enable-shared
      --with-sitedir=#{HOMEBREW_PREFIX}/lib/ruby/site_ruby
      --with-vendordir=#{HOMEBREW_PREFIX}/lib/ruby/vendor_ruby
    ]

    args << "--program-suffix=#{program_suffix}" if build.with? "suffix"
    args << "--with-out-ext=tk" if build.without? "tcltk"
    args << "--disable-install-doc" if build.without? "doc"

    paths = [
      Formula["libyaml"].opt_prefix,
      Formula["openssl"].opt_prefix,
    ]

    %w[readline gdbm].each do |dep|
      paths << Formula[dep].opt_prefix if build.with? dep
    end

    args << "--with-opt-dir=#{paths.join(":")}"

    system "./configure", *args

    # Ruby has been configured to look in the HOMEBREW_PREFIX for the
    # sitedir and vendordir directories; however we don't actually want to create
    # them during the install.
    #
    # These directories are empty on install; sitedir is used for non-rubygems
    # third party libraries, and vendordir is used for packager-provided libraries.
    inreplace "tool/rbinstall.rb" do |s|
      s.gsub! 'prepare "extension scripts", sitelibdir', ""
      s.gsub! 'prepare "extension scripts", vendorlibdir', ""
      s.gsub! 'prepare "extension objects", sitearchlibdir', ""
      s.gsub! 'prepare "extension objects", vendorarchlibdir', ""
    end

    system "make"
    system "make", "install"
  end

  def post_install
    # Customize rubygems to look/install in the global gem directory
    # instead of in the Cellar, making gems last across reinstalls
    config_file = lib/"ruby/#{abi_version}/rubygems/defaults/operating_system.rb"
    config_file.unlink if config_file.exist?
    config_file.write rubygems_config

    # Create the sitedir and vendordir that were skipped during install
    ruby="#{bin}/ruby#{program_suffix}"
    %w[sitearchdir vendorarchdir].each do |dir|
      mkdir_p `#{ruby} -rrbconfig -e 'print RbConfig::CONFIG["#{dir}"]'`
    end

    # Create the version-specific bindir used by rubygems
    mkdir_p rubygems_bindir
  end

  def abi_version
    "1.9.1"
  end

  def program_suffix
    build.with?("suffix") ? "193" : ""
  end

  def rubygems_bindir
    "#{HOMEBREW_PREFIX}/lib/ruby/gems/#{abi_version}/bin"
  end

  def rubygems_config; <<-EOS.undent
    module Gem
      class << self
        alias :old_default_dir :default_dir
        alias :old_default_path :default_path
        alias :old_default_bindir :default_bindir
        alias :old_ruby :ruby
      end

      def self.default_dir
        path = [
          "#{HOMEBREW_PREFIX}",
          "lib",
          "ruby",
          "gems",
          "#{abi_version}"
        ]

        @default_dir ||= File.join(*path)
      end

      def self.private_dir
        path = if defined? RUBY_FRAMEWORK_VERSION then
                 [
                   File.dirname(RbConfig::CONFIG['sitedir']),
                   'Gems',
                   RbConfig::CONFIG['ruby_version']
                 ]
               elsif RbConfig::CONFIG['rubylibprefix'] then
                 [
                  RbConfig::CONFIG['rubylibprefix'],
                  'gems',
                  RbConfig::CONFIG['ruby_version']
                 ]
               else
                 [
                   RbConfig::CONFIG['libdir'],
                   ruby_engine,
                   'gems',
                   RbConfig::CONFIG['ruby_version']
                 ]
               end

        @private_dir ||= File.join(*path)
      end

      def self.default_path
        if Gem.user_home && File.exist?(Gem.user_home)
          [user_dir, default_dir, private_dir]
        else
          [default_dir, private_dir]
        end
      end

      def self.default_bindir
        "#{rubygems_bindir}"
      end

      def self.ruby
        "#{opt_bin}/ruby#{program_suffix}"
      end
    end
  ...
  ...