I do have a mix now (of SVN and installer code/units), but it seems to be working OK, and I've already imported the library I needed.
I'll use this to check whether it will be possible to do what I want to do with Lazarus/FPC and then start again if it proves to be viable.
Great! If it works then no need to start over again.
Yes, different Lazarus sources can easily co-exist in separate directories. I was just over-cautious with my advice because people are having so many problems with installing and running it.
Most problems originate from trunk snapshots. They include FPC compiler. Then people install other compiler version without knowing how to configure them. Then they copy a new snapshot on an old one...
Updating a snapshot from SVN is one common experiment, too, because "the sources are already there". Sometimes it feels unreal how many problems people can create with installing and running Lazarus.
I personally can recommend trunk directly from SVN. There is also a Git mirror available.
A revision control system lets you update quickly and keeps network traffic low. It is the only way to go if you report bugs and want to test their fixes immediately, or you make patches yourself.