I can not see the harm in over size bases. It seems it would be to your disadvantage most of the time.However, wining is everything to some people. I can see someone trying to get you disqualified over it.
I have a lot of my old figures based for WarHammer. Several of my friends like games that use different base sizes. Rather than re-base I have made some new bases with sheet steel centers. I just switch the figures back and forth. The figures are already based on magnets.
The spider base problem. I think you can address the same way. I am not going to enter GW events so it won't mater much to me.
40mm magnetic base bottom with 40mm sheet steel base on 50mm base.
I am going to use non GW toy spiders but 40mm magnetic base bottoms will work on the 60mm bases too.