I wouldn't recommend a Royal Pleco
in that tank. It's a Juwel Rio [or Vision] XXX, and even the biggest RIO 400/Vision 450 isn't big enough for an grown out Royal Pleco. If you still insist that you have a Royal in there, you'd need to add some wood, because that's what your Royal should feed on. Yes, it does take YEARS to grow them to full size, but they need big tanks when they are fully grown, and it's much easier a problem to find a big fish to fit a big tank than to find a big tank that fits a big fish, if you see what I mean. Once you've got your 6' x 3' x 3' tank, you can get the big panaque. This is aside from the fact that the ideal conditions of this fish is completely different from the rift-lake cichlids that you want to keep in the same tank. Paanque's want soft, acidic water, and your rift-lake cichlids want hard/alkaline water.
Synodontis are much better match if you pick one from teh same conditions as your cichlids. There are several Synodontis species from Malawi and Tanganyika. Malawi ones are very rare in the trade, so Tanganyikan ones are almost certainly what you have to go for. The most common recommendation would be
or
. They are reasonably sized ones too, so you won't have to buy a much bigger tank when they grow up...
Also read
this article on how to spot hybrid species. You DO NOT want to buy a hybrid...
As for algae-control, you could get a
, assuming the cichlids aren't eating the algae by themselves. They are hardy and will withstand the high pH/hard water in the rift-lake tank, as well as survive/hold it's own against aggressive rift-lake tank-mates.
--
Mats
sorry it took so long to get this picture up 
/g/s/1.jpg)