From expreience GBR's will very happily live with both dwarf neons and Bristlenose plecs.
To be honest they'll get on with a lot of community fish. They were probably ramming your' Bolivians because they were too similar.
I have kept the German Blue rams with Bristlenose with mixed success. Some of the Bristlenose I have kept showed a fondness for eggs and this drew the ire of the rams who at times were fearless in defense of their eggs and at other times,,they chose to simply abandon the eggs or eat them.
Would be worth a try but I might try and get large specimen of Bristlenose as opposed to a very young one.
Don't believe there are a whole lot of community fish that appreciate the warmer temps that the rams appreciate.
Most who keep the Rams ,keep them in average temp of 82 degrees F.
When kept at cooler temps,these fishes metabolisim slows down, and they often become more suceptible to disease much like Discus who also enjoy(need), warmer temps.
Would agree that cardinal tetras and rummy nose would be good tank mates for they seldom venture to the lower areas of the tank that the rams prefer and enjoy nearly smae water parameters.