Hello, @Maky,
1. For making your products at the same height, please copy and paste the below given CSS code to XStore >> Theme Settings >> Theme Custom CSS >> Global CSS and check back your site after removing the browser cache.
.home .content-product .product-title a {
height: 75px !important;
}
2. You are not able to choose some of the products to add to the cart on the home page because those are variable products, and for turning ON the add to cart for the variable products, you have to show the variations on the Shop/Home pages and for that, you have to use the swatches.
You can turn ON the swatches Option from XStore >> Theme Settings >> WooCommerce >> Shop Elements >> Variation Swatches: https://postimg.cc/JyjqGWvR
Regards 8Themes Team.