This is a really excellent thread with lots of good suggestions.
One thing I might add is to remain adaptable and flexible once you start encountering hostile forces. All of this is good to get your army going, but you will probably need to alter it along the way for the concrete situations. I have experienced this in two ways.
The first was when I tried my first planetary assault. I got my ass kicked, but that was probably mostly due to using too few troops, so I started rebuilding the army to prepare for the second attempt. However, another thing I noticed was that the planet actually had a terrain other than barren. It was an ice desert. Now, ice deserts to not provide a targeting malus to your own troops, so all I had to deal with was the targeting penalty from forticiation, but that is still a lot. So I decided to design and build a selection of troops with desert and extreme temperatures specialization to increase their hit chance. This took up, perhaps, a quarter of the new army, and had increased CAP after I had gathered some actual intel during the first assault. These then became my 'Arctic Regiment'. I still use them for other assault, since I have them, but they were designed for a particular situation.
The second was after a successful assault, but one which lasted far longer than I wanted. Going through the intel, I could see that their troops had better weapons tech than me, so even if I hit a lot of troops, the kill rate was crazy low. For several reasons I have gotten behind on weapons tech and do not see myself catching up any time soon, so instead I have designed a new formation for the next planetary assault. These consist mostly of LAC and MAC, which have greater damage and penetration than CAP/HCAP and more shots than MAV, so hopefully they can get a few more kills against a technologically superior enemy. Still, this is only a supplement, so a single formation of 5.000 tons per 125.000 tons division.
Finally, I would add some of my own designs, which are rather different. Mechanically they fit into the templates others have said, so this is just to give yet another example of how a thematic approach to your empire can show itself in ground unit design.
My current empire is based out of a jungle mountain planet and they have taken their close connection to nature with them into outer space. In addition to the basic infantry unit (Hunters; PW) most formations are also accompanied by a species of canines native to Aldor. These Plasma Hounds (INF; PWL) are ferocious and a formidable force. Fortunately, they also breed fast, which makes them ideal for filling up the ranks. I mostly use these as cannon fodder. They don't really kill anything, but PWL take very little space so they can soak up A LOT of shots - even if they also die quickly to them.
From the deepest mountains in the dark caves, the Spiked Worm resides (VEH; CAP/MAV). These have been tamed and trained to form the backbone of the main raid parties, flanked by plasma hounds to distract incoming fire. In the far northern reaches, their cousin lives in the arboreal. The Frost Wyrm (HVH; HAV/HAV) is a major destructive force, but also rather slow. Infantry units can easily evade them, so their taskmasters train them to destroy tanks and other mechanized forces.
From the sky, the mighty Roc rains down upon the hostiles in swooping attacks. These are modelled using ground support fighters with auto-cannon and bombardment pods, but thematically they are large, winged creatures.