Always set a goal before you breed, even though just for mere experience (that's a goal too). That way you'd know what to look out for and later, whether you succeeded or not ..... in the long run it will give you better insight. And for me, I know how to guide you in a better way.
Yes. If both are extended red (including the marble) you should have ext red, red more to a cambodian or cambodian more to a red. and pale bodied cambodians, marbles in any of those patterns.
The DTPK should improve the fins of the red PK. Theoretically you will have 100% DT geno - most single tails, some DT with uneven caudal lobes, single tails with longer dorsal. If you're lucky you might get more rays in the PK fry. Selectively breed F1 to get the ideal ray branching/form.
What are your plans/goals? What color is the marble DT? Is it a DTHM or a DTPK? By red, do you mean cherry red or extended red?
If your DT is not a red or not a cambodian, it would be a shame because you will ruin both colors. You see red is very difficult to clean out - like iridescence colors. So if your DT carry irid colors you will have non clean red fry (red with irids).
I kept one of my plakats in a 1.75 gallon kritter keeper for several weeks and he did quite fine. Built nests and everything. For me, it's the length of the tank. He had more room to actually swim in the 1.75g but long kritter than he did in the 3 gallon tall hexagon tank. So I'd say if you find a nice tank that is smaller than 5 gallons and it works for you and the betta, go for it. :)