fw in formations?

I got a question for one and all. In the space marine codex all the formations options that are mentioned are only from the codex itself, what's your opinion of allowing fw units to be added into the formations that fit the same unit type?? Ex. Fw land raiders being used in the land raider formation..

You get iffy past Land Raiders. Is a Tempest subbed for a Stormtalon or for a Land Speeder? Can you sub a Sicaran for a Raider or for a Predator? Can contemptors be subbed for Dreads or only Venerable Dreads? (Important for Dark Angels).

What I'm going for is fw versions of the same tank being used in the formations from the codex, so a sircaran can't be used to replace a pred but a demos pred could, or an achillies being used instead or a godhammer, contemptors I would be 50/50 now that you mentioned it

My 2 cents.

Like for like

A raider is a Raider
A pres is a pres
Dreads- if it had BS/WS 4 it can be subbed for standard dread, 5 for venerable.
A javelin for a speeder/tempest for a talon.
Stormraven can be subbed for the Stormeagle and like.
A Sicaran can be subbed for a Pres Squadron. (So that one formation is a techmarine and 3 Sicaran, max)

I think discussion with opponent is the key. 7th edition is tricky enough that a hard and fast rule is almost impossible. Look at the Tau coordinated firepower rule and the discussion it sparked. I think it would fall under the "this is what I'm thinking" part of setting a game up.

Gone are the days of the pickup game.

Ya true I think I'll just leave it at that