Originally Posted by unstoppablex
i would avoid UV sharing between meshes just for the strong reason that if those meshes are not on the screen at the same time, you have wasted texture space because unused mesh textures are being loaded into memory because it shares it's texture map with something that is on screen.
as for the texture density, a quick way to solve your problem, if you downscale the pillars and it only uses a fourth of the 512 map now, make it a 256 map, which is exactly one quarter of 512
but one thing about texture density flow is that it doesn't have to be the same for all objects. it mostly applies to objects near each other, the floor etc. since a roof is out of someones reach, and the player sees it from a bit a of distance, the texture can be a bit smaller.
Will the pillars ever be seen separately from the building? The way I look at texture sharing is if it will always be part of the object you're creating sharing textures may not be so bad. However, if you can see yourself using the piece in another scene, level, area on it's own then you may want to make it a separate texture. Also, keep in mind the material types you're grouping it with on your texture sheet. Similar materials are easy to have on one sheet. However, if they're vastly different you may have a harder time creating a material as you may need to create masks to block out certain effects like reflections, cube maps, or where a detail maps tiles.
In regards to texel density, if you're making this for a portfolio piece, then you may want to have your texel density similar across all object since you'll be showing this off from different angles. From a gameplay perspective, you need to ask yourself, "how will the player see this?" If it's a piece that will be off in the distance or not easily seen, you can usually stand to have a lower amount of your UV space dedicated to that component.
Also, you may want to look into the way that detail maps operate as well. You can use tileable detail maps in your normal channel to bring out extra texture information when you're close to an object. If I laid out an entire road, I would have to use a sizeable texture map to capture the fine cracks in the asphalt as you moved closer.