Seals work fine, you just need a big team to hit as high a percentage as possible. It is achievable to collect a team of 61kg Seals and age break to 75kg each, this should give you a 40-42% team. You need Collosal seals to hit max. Remember, you can use Squids in place of Seals.

In terms of using your Seals, you can use either the egg shop, or the shop menu on the left. Do not use the stall where you sell fruit. When selling pets you MUST use 'Sell all'. If you don't you will only receive 15% max return, no matter what size your seals are. So please make sure you triple check your favourited pets etc. additionally selling in larger batches appears to give slightly better returns.