Hey mnakawake,
Thanks for your question and welcome to the community.
If you are using the wood just to make the forms for a slab to remove them later, then you can use essentially any type you wish.
The key for easy removal (remember though, that dried/cured cement will always outlast and be stronger than wood) is you can opt for coating the wood where it touches the wet cement via a lubricant, like cooking oil on the wood assist in prying off the slab.
Most slab installers will also opt for planning in using the wood in smaller, easier to remove sections, as well as small supports so installation will be effective, and the removal of the wood is easy.
This means you can choose a basic yellow pine or similar low cost wood, since it's just for temporary use.
Let us know if you have any further questions,
Joseph