Just purchased 7 extremely healthy looking cacti from B and Q

Unfortunately each cacti doesnt look like it has much room for growth - they
are currently in 3 or 4 inch plastic pots.

How much room should cacti have in order to feel comfortable? Should some
compost be visible?

Also, shall I bother feeding the cacti plant food? Or just water them once a
week or so with normal water?


Cheers


Tom