1st- Mental health is no joke. I used to put others first, be everyone's savior/ caretaker or keep quite about issues i was having never realized what it was doing to my body. The impact of that is horrible for your overall health. My wife has been getting me to talk to her about things, which is a struggle in its own, but I am getting a little better.  I truly hope it is getting better for you. 

2nd- I am not a competitive player. I care more about having fun and hanging out with my friends. I hate playing people get mad cause a dice rolled bad for them but they are still winning by a lot. I rather do anything else then be playing that game. So I say build what you want they are your toy soldiers and have fun with them.

As an example:

• Did I need a Captain Sargotta from the Masters of Maelstrom box?
  • No.
• Do I play Chaos.?
  • No.
• Is she going to make an awesome Strike Cruiser Captain for my Howling Griffon's?
  • Yes.

On of other Projects is switching out Thunder Hammers for Swords one on my Terminator Assault squad just to make them more unique. Rules wise they are just a Terminator Assault squad with Thunder Hammer and Storm Shields. 

So follow your dream of building YOUR Army.  Ignore the naysayers. 

3rd- Assassins, Sisters of Silence, and Ordo Xenos Inquisitor all would be a great fit.  My suggestion as this is Webway Strike Force the Inquisitor would have to be Puritan because of the Custodes and Sisters of Silence, so no xenos tech. ( I have not read the Master of Mankind so I do not knwo if any of the Imperials used xenos tech). A But this is also YOUR army so if you want xenos tech go for it. 

• A Knight- I think a Knight would be a great addition as it could bring heavy guns and draw fire as your going to be outnumbered constantly as it is. 
• Sisters of Silence- Bolters or Flamers variants. small squads i would not go more than 3 squads
• Ordo Xenos Inquisitor another way to get some heavy weapons and bodies with some Imperial Agents
• Navigator just for the psyker ability. 

4th- Colors- If I was doing the force

• Custodes- Gold Armor, red Cloth/ Plumes
• Sisters of Silence- Black Armor, Red Cloth/ Plumes
• Inquisition- Red Cloth, black and gold accents
• Knight- White, silver, purple 

Thank you so much for spending time to reply to my post. You've helped me out more than you know.

I've recently been prescribed medications for my anxiety and panic, but they say it will be weeks before I notice a real difference.

I have a terrible time making decisions, so at some point after I've assembled these models and grabbed some Sisters, I'll come ask again for help trying to build a cohesive force.

Anytime!

Sending you internet support tokens @Lathe Biosas, what a great idea for an army.

I can't help on colour choices but @W.A.Rorie seems to have covered it!

I hope you can enjoy the project and it isn't overtaken by your anxiety.

Would it help to sketch out the colour schemes? I quite often attempt to sketch out a mini and then block in colours to get an impression of what it might look like and how adjoining panels interact. It can be useful if you aren't simply copying a Space Marine from a codex. 

I'm sure this will be a fantastic looking army, very impressive as a fully painted force - especially with a Knight!

I never thought about doing that. I ordered the new Custodes models, but haven't really thought about where the  Colors are going to go.

How do I go about sketching it out?

The easiest way is to go free hand.

This is a colour test so it could be as abstract as you like - a Custodes helmet is a triangle sitting on top of a circle for a face which is on top of a rectangle for the chest, etc

Or you could do a loose sketch of an area where colours interact. Here are my Kroot test heads

What I will do is create a template image - pencil overdrawn in black pen - that I can then trace on to paper multiple times.

For a more complex reference image to use as a template you could try tracing.

For example, find a reference image in the build instructions and put it against a window with your paper over the top. Hopefully there will be enough light for you to trace what you need.

You can even trace off a screen but that brings risks so I don't recommend it.

Unfortunately, my more complex examples are in my desk at work and I won't be back there for a week. 

I say just go for it and have a bit of fun!

Oh, and there was a recent WarCom article with alternative colour schemes and there might be some in the Codex too. There might be scans of old Codex on the internet too.

Edit: found more template examples