Unable to save at 300 ppi using aspect ratios.

"I cropped some to ratio and they were 72 ppi.

It's very frustrating!"

It does not matter, you can easily change ppi. The most important thig is pixel dimension not ppi which you can change without affecting anything on photo as described below:

Go to Image > Image Size. Uncheck Resample then change Resolution to 300 and lastly confirm on OK. 

"If I save a photo at a large size - say 20x30 at 300ppi - can I print all smaller sizes of the same ratio from that saved file??"

In essence, yes you can unless your print provider explicitely ask for exact dimensions when printing. Images must be scaled down before printing and in many scenarios that can be done automatically including when printing directly from Photoshop. If you want more control then scale down images before printing and look at 100% magnification. Where you are going to print images? 

Do you know that as 8" x 10" images has a 4:5 aspect ratio.   That a 300PPI resolution  is a  printer setting.  That set the size pixels are printed  300PPI pixels are  are 1/300"x 1/300"  there are 90,000 pixels per square inch.  To print an 8" x 10" print with a 300PPI resolution you need havet an image that has 2400px across by 3000 px high  you need 7,200,000 pixels  four your image.   To print the image an other aspect  ratio you need to crop the image to that aspect ratio. You need to delete some Image content. Then scale the crop for  the print size and print resolution you want the print to be. You will loose image content and images quality.