I have not had a Catalina and a Luxfer AL 80 side by side for several years. But if memory serves the heights and diameters are the same with only minor differences shoulder profile (assuming they are both the more or less standard S-80's and not one of the various other AL 80's). You should be able to set these up as doubles with any manifold and band set designed for 7.25" diameter tanks.
I am a little confused on your reference to Luxfer LP 72's. Luxfer did make an AL 72 but they were 3000 psi. and were basically a skinny AL 80. They are I think taller than a steel 72 and very slightly larger in diameter so I am not positive what your options would be for manifolding them together.
If we are talking about regular steel 72' s (But I am not aware of Luxfer ever making steel tanks) then they could also be paired as doubles as well using a band/manifold set designed for 6.8" diameter tanks. Capacity required depends on the course and the instructor but many courses requiring doubles specify a minimum of 130 cu ft total and a pair of steel 72's at 2250 psi just make that minimum standard as they provide 65 cu ft each at 2250 psi (and 71.5 cu ft each at 2475 psi.)
The LP 80's are a great option as they are not overly heavy and offer an honest 80 cu ft at 2640 psi. They will work with the same 7.25" band/manifold set up as an AL 80, but the pair would be about 10 lbs more negative than a set of AL 80's.
Another option is to go with a set of travel bands and use the tanks as independent doubles. It is a very flexible and travel freindly arrangement and is as redundant as a set of isolator valved doubles and cover some failure modes that a non isolator valve manifold does not. They would also theoretically allow you to mix and match different capacity tanks, although this could create some interesting and complicated buoyancy and trim issues.
On the negative side, independent doubles do require better gas management skills as you need to switch regs as you breathe each tank down 1/3 at a time. The task loading is pretty minimal for an experienced diver even at depth but it is often cited as a potential issue and for this reason independent doubles are currently (if in my opinion somewhat unfairly) out of favor with most tech instructors. I'd check with your instructor before pursuing this particular option.
In any case, however you configure your tanks you are dealing with different tanks and potentially with both singles and doubles configurations and will need to be aware of the differing bouyancy traits and adjust your weight accordingly for each setup.
You will also need to be careful with wing selection as you need one that will offer the lift capacity required for the heaviest of your configurations. Some wings adequate for doubles to not always work well for singles and you may find yourself needing two separate wings.
I really reccomend you talk to your instructor about these issues and get their opinion. Most have pretty firm ideas on what they want in the course in temrms of equipment configuration. Some will also admit that what you do after that is your business but that at least after the training you will be able to make an informed opinion.
40 cu ft is often preferred, but unless you are an air hog, the AL 30 will work well for deco gas. The small diameter makes it very comfortable when slung as a stage bottle.
Personally, I dive with steel 72's exclusively (as I own 20 of them). The depths here are limited to 150 ft and, having very good air consumption, the 140 cu ft available is more than adequate for the types of dives I do using a rules of thirds for dive planning and gas management.
I also go against the grain and use independent doubles as they make sense for the dives I do. But...if I were going to dive in caves or do a penetration of a deep and large wreck like the Andrea Doria, I'd go with bigger isolator manifolded doubles with no hesitation. A diver has to know his limitations regardless of his configuration.
I also use an AL 30 for deco gas (normally filled with Nitrox 50, for added saftey rather than accellerated deco) and again with good air consumption and relatively short deco obligations it is more than adequate.
