Hi,
The size of the file and the number of products is not really a problem for HikaShop.
The only problem I see is the imageURL column.
If you use it as the "images" column, it means that HikaShop has to download the image of each product during the import to store it locally.
It's impossible to do that for 7000 products without hitting at least the max execution time limit of the php.ini of your server.
I suppose that the issue you got with the import of the big CSV file comes from that.
You could download all the images in the images/com_hikashop/upload folder and then only keep the filename in the images column of your CSV so that it wouldn't hinder the import process.
The message "product not found" doesn't necessarily means that there is a problem.
If you didn't specify the category in the categories column, the products will be added to the main product category of your shop if you didn't delete it.
So check there, you should see the products imported.
You also didn't mention if the report system of the cron task helped you. Did you get the email notification with the message from the CSV import plugin saying that it tried to run the import of the small CSV file ?