Well sometimes we have to put them in the rear, but it should be avoided. 110 degrees from the front axis is the suggested angle. Honestly I don't feel surrounds are super important. I mean it's nice to have good surrounds, but subs and mains are far more important. In fact I always suggest using cheap surrounds until one can get there subs, mains, etc. When I had my Beta 20 and 250 setup
Any Infinity Primus or Beta Series speaker will have the same tweeter as the beta 20 and will match very close in Timbre and would be ideal, but most can be bulky.
100- 120 is the recommended angle for side surrounds Though I've found 90 to be fine as well as 130.
Since I couldn't get away with using big speakers as surrounds
I run Kef 2001.2s as surrounds. Found them locally for 40 bucks a pop from an angry ex-wife.

They retailed for 200 a pop, but A4Less does sometimes sell them for 100. Check Craigslist(you never know

)
They do a decent job of matching the Betas and are a compact good looking ported design. Please note that my 2001.2s have more volume than the Beta 20s when the betas are off axis so you will want to level match with that in mind.
It doesn't mean the Betas are bad speaker they just tend to be less efficient off axis.
Of course my ears are different than yours so you may not like it as much.
With Audyssey EQ the sound stage with the Betas and KEFs was just plain sick. The only reason I changed stuff around is I'm trying to do more than just get great sound. I want to learn wood working and sound management.
I have personally used your front setup so if you have specific question feel free to ask.
Last words:Save for your subs you won't regret it.