try NOT to use blinking lights, a.k.a "lightstyles" in lightmaps. Because OpenGL doesn't support them well enough, so it does lag a bit.
Wait, what? I'm not sure what's wrong with the X3100 if that's the case. Granted, it's been years since I've properly mapped for goldsource, but back when I did I used lightstyles all over the place, and while it definitely impacted my compile times, it never introduced a framerate drop on any of the machines I used to map on, none of which were supercomputers for their time.
In my opinion, it would be a smart idea to make 1 texture with 2-4 resolutions, like: 1 floor texture, with a 128x, 256x, 480x and a 512x variation.
So, use the 128x for the non-detailed, non-reachable areas, and use the 512x (and/or the 480x) texture for the "obvious" parts, where the lightmaps should be detailed, for example.
An interesting idea, but I wouldn't go too overboard with it. The additional memory usage probably wouldn't be an issue to modern machines, but the compile tools do have a texture limit built in. I'm not sure if modern variants have upped it, (they probably have) but the original distribution of the tools defined it as 512 textures used. Using a bunch of different-sized variants of your world textures will drastically cut down the ones you have free for brush-based props and the like. Something to keep in mind when doing it, at the very least.
But rly? cuting on texture resolution just to get a low wpoly on SP in 2016? i dont think its a isue i 2016 even for goldsource.
Unless it's super impacting your performance, wpoly isn't really an issue these days. I don't think it's been an issue for a decade or so, so unless someone's maintaining an authentic 90s or early 2000s machine to play classic games on as a hobby, they should be able to keep up. As long as it compiles you should be fine.
why you say the texture resolution must be same as units?
I
think he made the assumption that you were working with a 1.0 texture scale. You can certainly use 0.5 (or 0.25 if you really want to try pushing the engine) if you want higher detail and crisper shadows, (As lightmap density is directly tied to texture resolution, unlike Source) just keep in mind that in goldsource, the flashlight's size is also tied to lightmap density, so the smaller the scale, the less light you can shine on it.
Other than that, I can't recall if there's a drawback or not to using rectangular textures - there might be a small memory or vram hit, but I reckon it should be acceptable in the few cases you'd need it. Other than that, just keep everything a power of 2 size like Shephard listed, and keep the scale as consistent as you can.
Bonus Archie texture tutorial.