1) There's a trick called a "horizontal portal" that you can investigate. If you place a series of portals based on the same relationship sided by side, you can set the first portal to display Row 1 of 1 rows. The second can be set to display row 2 of 1 rows and so on. If you have FileMaker 11, you can also get some interesting results by using portal filtering where the filter is different for each portal.
2) It depends on your layouts and what you mean by "make changes". If you modify data in a field on one layout, any other layout that can be used to view the records where that data is stored will show the same change. If you want to modify the layout's design, (add a button, change the width, color, text style of a field, etc.), then no, changes to one layout will not be reflected in another.
Whether you can pull different layouts together inside a tab control is problematic. If all the layouts refer to the same table occurrence in Show Records from of layout setup...., then yes this can be done, but all the tabs will work from the same current record, found set and sort order. It's very likely that this is not the case and thus it will not work for you.
Some developers simulate the effect of having different layouts under each tab by creating identical tab controls on each layout, but clicking the other tabs triggers a script that changes layouts as well as changes tabs. This creates the illusion that the user is just changing tabs when they are in fact also changing layouts.
1) See Phil's answer above. You can also put several portals side by side doing this to show records 1-5, then 6 through 10 and so forth.
2) If you mean can you have changes from one layout affect another, the answer is no. If you make changes to a specific layout and want the same changes made in another, you will either have to make the same changes or copy & paste the portion (note: you must have the same table reference ... Layout Setup - Show records from ____ or some of your fields and possibly buttons may not function properly). Read Phil's answer carefully above.
You can have some issues using tabs if you have required values or data checking. If you have the same field shown in more than one tab, you may seem to suddenly jump to another tab. Since this happens prior to any kind of save, the partial data you entered may not seem to show up in the other tab. Most people will try to re-enter the data in the new tab when it is still in the original tab and will cause the same error. You may also jump to an unexpected tab if a script takes you to a field that appears on more than one tab. Tabs are VERY useful, but designing them correctly is key. Hopefully Filemaker will allow a little more control over tabs soon.
When working with fields that are duplicated within tabs. It can be useful to give each such field an object name and then use go to object instead of go to field.