I asked that do we need to set a slab (for e.g.: if we need to take in transactions with fees not less than 200 sats/byte and not more than 600 sats/byte, do we need to (or can we) put it like 200 - 600 sats/byte transactions to include transactions within the mentioned slab only)?
Yes, you can include any transaction you want into your block.
A lot of people seem to not understand how a block is generated. Its not like you 'mine a block' and afterwards include your transactions into it.
I would suggest to read
https://en.bitcoin.it/wiki/Block_hashing_algorithm and
https://en.wikipedia.org/wiki/Merkle_tree.
I know about that thing, and I didn't ask whether I can include transactions after a block is mined, but can we decide our own fee (which was answered by DannyHamilton Sir that we can). Sorry if you misunderstood.
As well, if this is the case and we are allowed to choose our own size of transaction according to the protocol and decide not to include those low-fee/byte transactions, will they ever confirm?
If those low-fee/byte transaction will never be mined, they never will confirm.
Its fully dependend IF and WHEN a miner decides to include this transaction into his block.Im not sure what you exactly mean with 'we are allowed to choose our own size of transaction'.
Transactions can be up to 100kb in size (to be valid according to the network rules).
I'm sorry I used the wrong word there when I said "we are allowed to choose our own
size of transaction", but by saying that, all I meant was the same thing again that after we choose how much should be the min fee/byte and max fee/byte or a slab (as said before), how will those low-fee transactions ever confirm? Can't this be considered as a spam attack by miners' end too in order to break the rules by making transactions wait longer to make mempool congested in order to let others raise fee just for the purpose of profits? What if (a) miner/s never decide to take those low-fee (even 100 sats/byte) transactions in their blocks? Everything (POWER) is in the hands of miners and if they choose not to include low-fee transactions and only raise the fee ahead, will there be anything that might confirm them (even in future)? Or such transactions need to be repeated with higher fee (possibly double-spends) to get through?