Watering recently bought plants

I recently bought those two plants in a supermarket. Both were already flowering.




I guess these are M. spinnossissima and M. zeilmanniana.
Should I give them water becourse they are already in flower or should I wait as normal until april?

The first one looks more like M. backebergiana. If it was me, I would take them out of that peaty compost, clean the roots thoroughly from all remains of it (expect to loose a few or a lot of roots*) and plant them in a well draining compost. After that, you should wait a few weeks before resuming watering, carefully at first.

*sometimes peat clings to the roots too tightly and you need to wet them in order to remove all remains. If you go that way, put the plants aside for a few days, allowing the roots to dry and heal any wounds before potting them in dry compost.

I lost a few plants in the past after re potting them directly when I got them.

Repotting to completely dry substrate is very unlikely to kill your plants. It may be that some wounded roots caught some infection, but that is also unlikely to kill your plants if you have waited a few weeks before watering. On the other hand, watering a cactus straight after repotting is an almost sure way to get it in trouble.