When you buy templates and don't have the knowledge to modify them, you need to insure that you only buy templates that do exactly as you wish.
Find another template, or hire a developer or designer to modify the one that you have.
If you want to modify your existing template, you first need to make sure that it CAN be modified. Many purchased templates are only supplied in minified form, which is useless for making changes. Make sure you can get un-minified source files.
I can't guess what the developer meant by "it's just possible with jQuery libraries". That layout looks like it can be done with just CSS.
I don't understand what your links are. Are either one of them your site? Are either one of them demos of templates? Or just some site that looks like what you want?
The generic term for the type of layout in the second link is "masonry". There is a jQuery plugin by that name. But you will find many plugins and CSS files with similar names. They are all the same basic concept. It can be done only with CSS. In the past, it wasn't so easy in CSS, and so a plugin or other JS script was needed.
Many theme authors go to extreme measures to make it difficult to modify their code or CSS. Others do not.
If CSS and JS files are minified, they are next to useless for modification. (However, usually CSS overrides can be worked-out.) Some authors go to further lengths and use special "obfuscation" to make it even more difficult to make the JS understandable.
You have to see if you got un-minified files that you can modify.
A "theme" is really just a set of examples pages and some CSS and JS. So, it is difficult to generalize.
As well, if it is for Wordpress, then it includes some server-side things as well, which are outside of the focus of this forum. We deal with jQuery, not Wordpress.
I would think that a grid is fundamental enough that you need to pick a theme that already uses the kind of grid you'd like.
The best person to ask is the theme author! It is difficult to give any really meaningful answers to these kinds of general questions. The forum is most effective for answering specific questions about jQuery APIs, "why doesn't this work?", etc.