The Annual Parish Council meeting that must take place in May is the meeting where the Chair is elected and the membership of committees for the following financial year is agreed. It would be the correct time to form new committees, as is the case with my parish council, and to agree the appropriate terms of reference for them.
The reason for committees is to benefit the running of the parish council - to make the decision making process more streamlined and efficient. Simply, without committees, all the responsibilities would be transferred back to the full council for decision making. So there are no 'legal implications' or 'impacts on past projects' - it is just business as usual but a very, very long full council meeting!
The art in committees is to ensure that they have enough authority to act appropriately but that major decisions are still ratified by the full council. If the council have adopted the NALC standing orders with no major modifications, it will allow for this but it also needs clear terms of reference.
With regards to the task and finish groups, this is the route that my council are heading so that events and specific projects can be undertaken quickly but with full approval to be gained from full council before any commitments.
I would suggest that you need a planning committee unless the clerk has delegated authority to respond or unless Full Council discuss planning applications on a monthly basis. Our committee meets fortnightly.