As soon as you are using the Page Layouts just for what they're intended to (that is, assigning it to a new page), I think it's not necessary to explicitly add the content type to the Pages Library.
Just by adding the Page Layout to the "_catalogs/masterpage" folder as "GhostableInLibrary", and associated with the needed content type, will make them available when you create a new page.
<Module Name="PageLayouts" Url="_catalogs/masterpage" RootWebOnly="TRUE">
<File Path="PageLayouts\MyPageLayout.aspx" Url="MyPageLayout.aspx" Type="GhostableInLibrary">
<Property Name="Title" Value="My Page Layout" />
<Property Name="PublishingAssociatedContentType" Value=";#Welcome Page;#0x010100C568DB52D9D0A14D9B2FDCC96666E9F2007948130EC3DB064584E219954237AF390064DEA0F50FC8C147B0B6EA0636C4A7D4;#" />
<Property Name="ContentType" Value="$Resources:cmscore,contenttype_pagelayout_name;" />
If, for another reason, you need the content type to be available on the Pages library, you can attach it programmatically in the same "FeatureActivated" event receiver, following a guideline such as the one provided in MSDN: http://msdn.microsoft.com/en-us/library/aa543576.aspx