Initiative: We know they're there, we should have a plan

During session 2, which I'm still working on the report, the party came to a door with holes they could use to look through. Beyond the door was a room with some giant rats. They decided to open the door, go in, and to take on the rats. I determined that the rats were not surprised and had everyone roll initiative as normal. As a result of initiative a PC in the second row went first. Since he acted before the PCs in front of him could move, it limited his options.

With the PCs knowing what lay beyond, they could develop a plan of action. To my thinking this would cause the normal roll of initiative to be inappropriate.

Here is what I plan to implement for future sessions when the party wishes to take the time to develop a plan which includes the order in which each PC will act. The PC who will act first makes his initiative check. Every PC after will have an initiative value of 2 lower than the previous PC. I will roll initiative for the opponents as normal and slide them into initiative order based on their results.

Example: Say the party decides to go in the following order: Frocensin, Gleran, Anders, and Milo. Frocensin would make his initiative check, say he got a 14, then Gleran would have 12, Anders 10, and Milo 8. Then say I rolled for monsters in two groups: boss and minions. Minions got a 9 and the boss got a 12. So the order of initiative would be:

1. Frocensin (14)
2. Gleran(12) in case of tie, PC goes before NPC
3. Boss monster (12)
4. Anders (10)
5. Minions (9)
6. Milo (8)

So which ever PC acts first better roll well.