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I searched for a similar question, but the topic was over 10 years old, so I guess it would be better to start a new one.
I want to build some rough riders and got some AoS riders to convert them from GW, together with 35x60mm oval bases. I also saw some biker bases on their website with 25 x 70 mm.

Now besides of this project, I picked up some other minis of Fireforge Mongols that I want to include, that came with 25x50mm rectangular bases and also saw that Kromlech has 25 x 50 mm and 25 x 75mm oval HDF bases that are much cheaper than the GW stuff (4 € for 20).

Therefore my question is: what is your experience with rider bases? Would it be a problem if I just use the rectangular bases that came with the minis from Fireforge? And if they have to be rectangular, do they have to be 35 x 60 mm or is it legal to use 25 x 75 or 25 x 50 mm?
I mostly build for myself, but if I play sometimes in the future I don't want to give the impression of modelling for advantage.

i'd use 60x32mm oval as used by chaos marauder horsemen:
https://www.games-workshop.com/en-GB/Chaos-Marauder-Horsemen-2016
Although i used the biker bases as i did quite a few years ago. i don't think anyone would mind if i rocked up with these:

rectangular ones would just stand out these days imo.
if you're set on the hdf ones, i'd go for 25x75mm ovals.