Hi Bob,
I think that's a really good idea to welcome the single travellers (although perhaps not everyone would like it - maybe the rep could gauge the water). I've said in the past I would always go with JY as my first choice and TS if there was a tour I really like that JY don't offer.
I've been on 2 TS tours - Peru and Cambodia/Mekong Delta. Peru had just over 30 travellers. Most couples were very friendly. I got friendly with 3 other solos (tho technically they knew each other having met on previous trips). In Cambodia there were 3 solos in a group of 15. Thankfully this smallish group of 15 all got on very well. I think eating together helps - all our meals were included on this latter trip.
As you've intimated it's the luck of the draw and as you have said, if it's a tour you really fancy, give TS a go.
I reckon if you put it out there you were going to Romania, there would be a good few JYers who would join you. I can't believe this is still not a JY tour with many in the past on the forum saying they really want to go there.
Angie