Any chance of getting the multi-game downloads broken out into their individual games or having installer options to install only the user selected games from those included rather than just blindly installing everything?
(I'm assuming that the other multi-game packs are like RoA 1+2 where in just installs both. In this case that's fine as they're fairly small, while I'm think that the M&M pack could end up being fairly large. Most of these I install under linux on my notebook and almost always find myself in a disk space crunch, and expect this to continue until I upgrade the hdd & re-partition/install...)

Actually, in most cases, you CAN choose what you want to install - maybe this is an exception... Or youre just as blind as I was when first using gog installer :D

I can't speak for that game in particular but others do offer the ability to install just one of the games, etc. I don't recall off-hand if the choice is shown by default. If it isn't then click on the options button and you should see it listed along with the ability to change the install directory.

I'm quite sure most every multi-game package allows you to choose what to install. The RoA games MAY be an exception, but you should check it again.
However, as Bansama pointed it out, you have to click on "Options" first.
In the case of the MM6-pack (just like with every game affected), you can even choose wether to install DOSBox or not.