I have used OBS to stream other games but I am thinking about streaming poker. I am having a few issues with setup but the major things I need help with are:

1) I use a third party program called a HUD that tracks live stats on my poker client (I can see it on my screen but it does not show up on stream). I am using Window Capture because i do not want to use display capture. Is there a way for the HUD to display on stream with using Window Capture?

2) What is the best way to get a chat overlay on stream? I tried using streamlabs but the widget is so small and the only way the chat is readable is if I expand the window size too large so I cannot fit the poker client at a large enough resolution. I wanna do something like All In Pav's setup:

Just to comment on your 2nd question; you can easily change the text size of streamlabs and imo it's pretty much the best option out there.
Remember you can increase it ratio wise and also height/width in OBS itself. And the text size from the Streamlabs browser UI.

You could try KapChat as well, but it's getting pretty old and doesn't have quite as many features as streamlabs does.

I see the URL at the top of the widget though and when I enlarge text, it goes off the screen if I place chat bottom right like screenshot above.

You will need to adjust the overlay from OBS accordingly, to match your text font size provided by Streamlabs. Made a little example illustration below.
And for the text resizing, go to your Streamlabs page go to Widgets -> Chatbox -> Font Size and use the slider. I assume you got this part down though, from the sound of it.

From there you just play around with the settings until you get a desirable match, which hopefully doesn't go off screen. Took me a while to get it right as well.

Ok is there a way to captured layered windows using window capture?

If it's just another program.

Try just using game capture.

If that have u tried window capture on both and if there is a solid color where it's suppose to be see through. Just use a chroma key to remove it