parachain is it ok to have
collator in one node? If not why? I'd love to hear some deeper explanation.
It depends of who can access the RPC part but mostly it is not a good idea if other people than you can access it.
The role of the collator is to produce blocks, at a very specific moment (when receiving a new relay block) for a very limited amount of time (currently limited to 500ms).
If during the time the collator is producing the block, most of the computer resources (not only CPU, but also memory, I/O, locks...) are taken by some RPC requests, it will reduce the probably to produce a full block and also reduce the probably to have your block sent in time to the relay.