Whether you can use the plate you already have for the gantry sides really depends on how flat and straight it is. If it is not flat enough you will put some kind of loading in the X axis carriages and they may bind or at least have a shorter life.

If you have the plate already then try lightly clamping it in place and see if the carriages move freely. Maybe you can get away with a bit of shimming.

Ideally I would go for the tooling plate myself as it is not much more than the regular plate and is much, much flatter.

One of the earlier questions was 'is tooling plate a type of T6?'. My understanding of the types aluminium goes as follows-

Aluminium 1000 grade (1xxx)
This is good for bending into shapes and is usually what the thin stuff if made from. But it is a fairly pure aluminium (99.5%) and likes to weld itself to the cutter, so flood or heavy spray coolant is required. Plus I have been recommended by experienced machinists to use carbide cutters.

Aluminium 6000 grade (6xxx)
This good for machining, and is what the general purpose thicker stuff is made from. It is only 97.9% aluminium with the rest coming from various beneficial ingredients.
Of the 6000 series you will often see 6061, which is a specific and popular product from that range.
Of the 6000 series (and maybe others as well?), -T6 (e.g. 6061-T6) refers to the product being heat treated and aged, which improves the material properties. Again this is a popular choice.

There are other grades, 2000, 3000, 4000, 5000 (good for corrosion e.g. marine use), 7000 (stronger), 8000. I've not knowingly used these other grades and have certainly not ordered them for workshop use.

Eco-cast / tooling plate / mic-6
Happy to be corrected but tooling plate / ecocast / mic-6 are the same basic thing to my mind - machined fairly flat and good to use as-is for DIY CNC machine.

So in short, for main construction go for 6061-T6 to get the machinability and other properties you need, and where flatness is important go for the ecocast/tooling plate/mic-6 versions of this grade which have been machined flat for you.

Hope this helps, and if anyone can add to this or correct me etc. please do.


p.s. Having a peak at Wikipedia to see what they say about it all gives a whole lot more info and here is the link if you are brave enough:
https://en.wikipedia.org/wiki/Aluminium_alloy