Yes, you can diversify your investments across any combination of ISAs. Please make sure you do not exceed the maximum allowance of £20,000. For example, you could split your allowance across one Cash ISA, two Stocks and Shares ISAs and three Innovative Finance ISAs.
For previous year ISA investments you can have as many different ISAs as you would like as well. There is no limit other than the amount a particular ISA manager will allow you to transfer. For example you can have multiple Cash ISAs, Stocks and shares ISAs and Innovative Finance ISAs, as well as a lifetime ISA if elligible.