Archers will happily spend the rest of their lives eating floating food, just like other fish. That said, you're missing out on a treat by not watching them spit. I trained mine to spit when changing the water. I'd lower the water level by a few inches, and then after the fish had settled down, stuck some prawn chunks onto the glass. The archer would then spit off the prawn, I'd have some fun watching, and then I'd add the new water. There's a trick to training them, which I describe
here.
Silver dollars are about the same size as archers and should make fine companions. I've not tested that combination, given mine was a brackish water species, but since silver dollars are not very different to monos, I'd guess they would be just as good. Silver dollars are hyperactive and pretty big, even the smallest is about 10 cm long. So I think you need to be looking at a tank certainly more than 120 cm (4 feet) long once the fish are mature. Perhaps a bit smaler for juveniles. A 90 cm (3 feet) aquarium would be, I feel, just too small.
A big tank would also let you have space to add some catfish. Adult Corydoras would be fine with an archer, as would smaller pims and dorads, Synodontis, basically anything in the 8 to 15 cm length bracket. Archers don't really like being kept with substantially bigger fish unless those fish are totally docile (gar and Colombian sharks work fine, cichlids generally don't).
Cheers,
Neale