How should I determine a ratio between malts?

For a 24L batch of extract lager beer I often use this recipe. It can be modified as wished but it gives a guide to what can be done. The hops can be changed for any similar amount of "nobel hops" but this mix gives a light clean hop taste and mild aroma.

40g Hallertauer-Herbrrucker boil for 45 minutes

15g Tettnang boil for 15 minutes

2.5Kg extra pale DME (This might be split 50/50 for wheat and pilsen extract, not a problem. 2Kg will give a lighter beer and 3Kg will give a beer with more "mouth-feel" and higher ABV)

0.5Kg Glucose (or more if you like a higher ABV)

I use Safale23 lager yeast but Safale05 would work too or try WB-06 for Bavarian wheat or K97 for a less attenuated Belgian wheat beer style)

fermented for 14 days cooler than hotter. 15C - 17C is not a bad temp range. It can go colder but may take longer. Try not to ferment at 20C.

150g dextrose priming sugar before bottling (for approx 48 bottles)

At least from the ingredients you've listed, you might want some more extract if you're making a 5 gallon batch. From the extract kits I've done, there is at least one pound of extract per gallon. I would recommend looking at other recipes of the size you're planning to get a handle on it that way. LME and DME both get used a lot, so knowing how to convert quantities of them for your recipe can be helpful (using brewing software to do the calculations for you is also very common/helpful, especially if you're not doing 5 gallon batches).

A pound of Carapils is maybe okay in a 5 gallon batch but you would want to use much less if you're making something smaller. As Robert mentioned, keeping your specialty malt (Cara- malts, chocolate malt, roasted barley, etc.) to at most 10% of the overall malt bill (by weight) is probably a good rule of thumb while you're getting a handle on the brewing process and figuring out what you like.

As for mixing the DME, I find it useful to look at the Breiss website to see what the extracts are made with. While you don't to concern yourself too much with this as new homebrewer, it might give you an idea of how many different things you'll be adding and it what percentages. As you learn more about brewing and the different ingredients and process you can get a better idea of how to use your extracts and what beers you can make with them. Given what you have I would probably just add all of both but if you end up getting more extract you might want to just get more of one (probably the Pilsen?) and let that be the bulk of the fermentables.

As for including some rice syrup solids, I could maybe see adding some to lighten your beer a bit but I'd encourage you to forgo it at first and see how close you get to what you're looking for with malt extract and then adjust from there.

Also, if you are doing a small batch, I'd be careful about using the whole ounce of hops for your full boil, you will want to scale that down with the recipe size as well.