Congrats on starting to save early! Especially congrats on deciding to educate yourself on the subject, I really wish I'd done so at your age!
That said, the first thing you should do is come up with a decision of where you want the basic parts of your portfolio to be in the long run. This will require you to come up with an idea of what your risk tolerance is. Since it's almost expected that you'll see stocks drop in value by 50% at least once in your investing career one way to try to get an idea on your risk tolerance is try to imagine what it'd be like to watch a $200k portfolio that was entirely stocks drop to $180k, then to $160k, then to $150k, then to $130k, then to $110k, then to $100k then to $80k, etc.... when do you think you would be scared that it's never going to drop dropping and be seriously tempted to sell it all and run? If it'd be at $150k then a 50/50 equity/bond split would be as much risk as you'd want to take. (if 50% of the portfolio falls 50%, your portfolio is left at 75% of total) If at $100k or less you might be ok with 100%. This isn't a precise thought experiment by any means, but it may help.
Once you've decided on your equity/fixed-income split based on both
your risk tolerance and
your need to take the risk.... then you start thinking about things such as how much do I have in domestic equities vs international.
I'd suggest taking some time and settling on a stock/bond and domestic/foreign allocations, i.e., what you'll need for a 3-fund. There's no rush on this, a month or 6 now isn't much of anything given the decades ahead of you. Feel free to keep throwing savings into the total stock market index (or maybe switch to a total retirement) during this time; it won't hurt anything
I'd almost say stick with total stock as if you get to see a drop or two of different sizes with it it may help give a feeling for your risk tolerance. A graph of the stock market can feel different when it's suddenly a graph of your money
Now, you mention small cap; this is getting into Fama and French's 3-factor model of investing. It's talked about a lot around here with people on all sides. I wouldn't decide to tilt to small or value until you've done a lot of reading on the subject. Same thing with REITs, read up before you decide on them. I didn't do as much reading as I should have and now find myself second guessing myself on my decision regarding REITs.
Personally when I started I spent a good month poking at the stock/bond domestic/international thing and running example numbers through historical regressions with different stock/bond mixes and reading up on stuff before I felt I had a decent idea of where I wanted to be with just these things. I then, because I like complexity and making things hard on myself, decided I wanted to read up on small and value factors and slice and dice and tilt and create a portfolio that requires a large spreadsheet to really track things. That took me another 6 months of glorious fun to get where I felt it was implementable.
Point is, there's no need to rush into everything (or even rush into anything). If you go with a nice simple 3-fund (or heck, even 1-fund total-retirement) portfolio now, it's easy to in a year or three decide that you now understand small and value factors and the correlation arguement for REITs or whatnot and that you want to add something to the portfolio (it's also easier to add tilts when you have more accumulated because of fund minimums). It's far more painful to struggle with a more complex portfolio for those initial years and then decide that now that you really understand more about these things that you don't want anything to do with them, and you're beating yourself up for getting in the position in the first place, and... just bleh.
So short version: Start with the basics, it's easy to add complexity later if you really feel you want it after you've had a chance to learn and experience more.