This post is more so I don't accidently hurt anything just doing what sounds right to me (like last time when I bought 2 turtles because I thought one would be lonely, btw don't do that unless you have A lot of room. If one ends up being male and one female )..

Hello I am setting up my new 29 gal aquarium. I currently have a 20 gal with a bunch of guppies in it that will be moving over to the 29 gal. I want to change from the blue and black gravel setup I've had (I really wish I never mixed those two bags of gravel together) and switch to either all black sand, gravel, or a mix of the two). I plan on getting RCS and I read they like dark colored substrates.

I will lightly plant this tank, in the future for now I just want to get it set up. I also don't want to go through a full cycle so is it ok to put some gravel in a cup or small bucket and place it in the new tank so I can easily pull it out later after the BB has spread around a bit?

EDIT: I forgot to subscribe to my own thread when I was posting to know when I get responses. Is it to late? I don't see a button now.

Normally I would just take the filter out of the 20 but it will still be in use and I don't think I should take my filter from the guppies. The 20 gallon will eventually become the sump for the 29 gallon I just need to figure out how I'm going to attach them to each other.

So to make a long story short:
Can I mix sand and gravel together?
Should I just choose 1 type of substrate and not mix them?
Will I have a problem in the future with plants rooting in the sand?

You can mix sand and gravel. The blend of sizes looks pretty natural, IMO.
The way I do it is to use more sand and less gravel, perhaps 75% sand + 25% gravel of mixed sizes up to about 1" diameter.
Put the sand in first, and plant. Then add the gravel sort of in drifts and bands. Not evenly mixed. Think of how a wild stream will have some calmer areas where the sand can settle out, and some more active areas where the top layer of sand is washed away. I like to add a few larger rocks, too, perhaps 2-3", but not many. A little group over here, and a few more over there. Very easy to separate if you later decide you do not like the look.

When you are starting a new tank you can share some bacteria with the established tank to jump start the population in the new tank. Or you can buy a bottle of bacteria. Just make sure it contains Nitrospira species of bacteria.
The goal is to have enough nitrifying bacteria to handle the waste from whatever amount of livestock you will have in the new set up, without stealing so much from the established tank that the old tank shows an ammonia or nitrite spike. I have removed as much as 25% of the filter media from a well established tank to help get a new tank seeded. There is also bacteria on the substrate, and on all the other surfaces in the established tank.
If it will be a while before you stock the new tank, this is good. You can continue feeding the bacteria with ammonia until you are ready to stock the new tank. Then a big water change to remove most of the nitrate.
If you want to stock right away, after set up, then I would use a small amount of filter media, and a bottle of Nitrospira for a really fast cycle.

Pick up a bag of Black Diamond blasting media. You can pick it up at Tractor Supply if there's one near you. It's around $9.00 for a 50 pound bag. It's black, easy to plant in and super cheap. Add in some DIY root tabs and you'll be good for some easy grow plants.

Thanks for the info. I picked up 25lbs of black aquarium sand I will use that first, most of the gravel in my 20 gallon is black so I'll try and take some out of there to put on top and help with spreading so BB. I also picked up some java moss, when I'm done cleaning gravel I'll try and attach it to some decorations in my tank.

I'll also be taking the foam out of my established tanks filter and put it in the new tank.

Dang I already picked up some aquarium sand from petco. I might still go grab some from a Tractor Supply shop, I want to change the sand in my 100 gallon turtle tank its light brownish right now. I'll have to look up and see where the closest one is so I can grab 3 or 4 bags.

Thanks for the help everyone.