If you have a front end loader, I would not put fluid in the front tires. Use your loader for ballast. You will get far more added weight from that, than you will with those small front tires. I would not foam fill them either as now you basically have a solid tire that has a punishing ride at a cost that will scare you. I would only do that if you have a lot of flats.
The rear tires are so small, that I don't think the benefit out weighs the cost. Also, if you do puncture the tire, you leak out out the liquid.
My advice would be if you need weight on the back to hold it down while doing front end work, then either hang a heavy implement on the back, or build a ballast box that you can pick up on the 3 point hitch and remove quickly as needed. This gives you the most flexibility with your tractor. If you need it light to do yard work, say mowing or spraying, or spreading fertilizer, then you can easily drop the ballast box and remove the loader and you now have a lighter weight doing less damage to the soil.
If you need to load gravel or push snow, you can pop the box and loader back on and you are good to go.
That gives you the most flexibility and lets you optimize your tractor for many different tasks.
If you load the tires, you have no flexibility to make the tractor lighter if the job at hand requires less compaction.
I want my tractor to be the lightest it can be to get job done safely and with the least amount of soil disturbance . If I am mowing pasture, i normally take the loader off, if I am picking up big round bales on the back, I put the loader back on and add the bucket to keep my front end down.
In my case, when I use the front end loader, a lot of time, I will just stab a big bale and use that for ballast while doing loader work. Obviously in your case the ballast box would be the way to go.
Good Luck and enjoy your new rig.