Yes, you can mix malts types. It's common in mini-mash recipes, for example, to use LME for some of the fermentables, and mashed grains for some of it.
Spray malt is DME, I think, maybe with some caramel malts (carapils) for flavoring and color, and probably comes in different grades from extra light to dark and maybe even wheat. I'd just treat it like straight DME when calculating the amount to use.
Sugar is 100% fermentable. Spray malt is about 80% fermentable, so you'll need 1.25kg for equivalent fermentability compared to 1kg of sugar. I'm practically certain it will produce a better tasting beer than one made with 1kg of sugar in it. If you add another 250 grams of the malt, you'll increase the gravity of the beer and you'll probably get a higher ABV, but you might also end up with a sweeter beer. So you might want to consider increasing your bittering hops or use a yeast with a higher attenuation. Personally, I like my dark beers to be on the sweet side.
As for whether to go with light, medium, or dark, that depends on what you're trying to achieve. The darker ones, especially if they contain carapils, are likely to have a roastier, toffee-like, or caramel-like flavor. If I was shooting for a dark ale, I'd either go with the dark spray malt, or I'd use the light malt and steep some crystal or roasted malts in a steeping bag to better control the flavor and color results.
