(Why is noone playing PTE?) Renamed : Now we have a good reason to play PTE

Hm. Apparently I don't need to.
Weird, I seem to remember needing to do it last time, I guess it was just because of the new launcher.

No one plays it because it's got game breaking bugs in the current version not found in the previous PTE. Selection icons are broken, you can't view buildings the other guy is building, and there are two grenadiers.

Can I use the experimental versions and the steam version side by side?

Yes.

I think the best way to use the PTE build is in a very directed manner. To wit: You agree with a few other PA players that you want to explore the balance changes in the PTE, so you deliberately patch to PTE and set up a game already knowing you'll have the players. In it, you do your own version of the directed testing I've been streaming, but with your own goals around learning something. Maybe you want to explore "what's the best time to go to T2?" with ideas like, "as soon as I could run the factory," or, "as soon as I run out of uncontested space," or, "Minute N," or whatever. Maybe you want to explore the orbital changes more. But it is probably something that is unique about the PTE environment at the time.

In this made up fantasy world where you follow my recommendations, you play a bunch of games in a row, trying to refine your self enforced strategy constraints (e.g. "help defend my territory with anchors above the battle lines"). After you do that, you post a discussion on the forums (I know you will). In it you first state raw facts about your experience that. E.g. "I tried to xyz. The first four times my economy stalled when I whatever. etc." and then you editorialize with your conclusions.

Then the usual suspects chime in with their favorite opinions like, "Your problem is the lack of unit diversity." or, "vanguards OP" or whatever. But some people offer some new unique perspectives, or even try to repeat your activities to see if they can validate your findings through actual game play, or maybe say they ran into the same issues but they found a way to deal with it. Fun!

I played it once, just because i was interested, but i got told that is was for testing and finding bugs, not to constandly play on without actually locating bugs or balance problems.

You're an optimistic fellow aren't you Meta!