Besides that, everything is compatible in multiplayer. You simply can not fly the planes which you don't own. On the bright side, you can jump to gunner's nest in big bombers and support the pilot, even if you don't have Gotha, HP or DFW C.V.
The RoF ICE comes with eight (+1 bonus plane) planes, which are the most iconic ones for the WW1: SE5a, Camel, SPAD 13, Fokker Dr.I, Fokker D.VII... The bonus plane is usually the Nieuport 17 which gives an availability to join servers with the "early" plane set. But most of them are the ones with the mid or late plane set.
OK, I own all the planes, because I bought the game one it was availble almost two years ago, so buying new plane once a month was not a problem for me. I understand a frustration from the new players who bought the game and if they want to have all of the stuff, they have to buy another 10 birds or so. But honestly, even if I own all the planes, i use only: SPAD 13 (stock), SE5a (stock), Pfalz D.IIIa (stock), Fokker D.VIIF (additional), DFW C.V (additional), HP O/400 (additional) and Gotha (additional). But keep in mind that I fly multiplayer only and my online unit has a bomber squadron. This is why I own the giants as well. For someone who is not interested in flying a bomber, the base back is OK. Not to mention that each plane is completly different and requires some learning curve.
BTW: The field mods are availble in the Store. They give additional equipment for the planes. They consist of reflector and collimated gunsights, water temp. gauges, fuel gauges, ASI, altimeters... The SPAD 13 and Albatros D.Va mods will be shipped for free to give a taste. Prices are from 1$ to 5,5$ for an addon, depending on number of modifications availble for each plane.
And in the couple of weeks we will hopefully see a 1.19 patch which will include new Career system which should be a blast
Lucas