(Spoilers) It should be possible to equip all the non-standard fishing gear at once.

[Glamorpuss]

Even with a fully-upgraded boat[ibb.co] you can't fit all the eeeeeevil fishing gear at once. My boat must be fully kitted out with dark forces - that stuff's the best way to fish! XD

The tendon rod is a C shape, which you can't fit beside anything else. There's not even one fishing rod or other gear, evil or otherwise, that is 1-by-1. (Unless you get the DLC for the sign of ruin.) Maybe the last space-upgrade for fishing rods should have a 3-by-3 space on one side, instead of both sides just being 2-by-4? I'm still missing a 2-by-2 contiguous space for the encrusted talisman.